1. Field of the Invention
The present invention is directed to the treatment with rare earth metal of refined steel melts for cold rolled, non-oriented silicon steel for purposes of desulfurization without the production of polluting smoke.
2. Description of the Prior Art
In a typical prior art manufacture of a cold rolled, non-oriented silicon steel, a melt is produced in an electric arc furnace or other suitable type of furnace. Typically, the melt is deoxidized either by furnace additions, tap additions or additions in any number of reladle operations. Calcium silicon has been determined to be an excellent desulfurizer and has hitherto been added to desulfurize the killed melt.
The calcium reacts with sulfur and floats to the surface of the silicon steel. This reaction produces objectionably large amounts of smoke comprising fine lime powder. Upon desulfurization, the melt may be argon bubbled or degassed (or both) to improve the cleanliness of the melt and to achieve temperature uniformity throughout the melt.
In recent years stringent pollution control laws have been enacted. In order to have their melt shops meet these laws, prior art workers have turned to the use of expensive pollution control equipment such as fume hoods and bag house systems to control the smoke produced by the calcium-silicon desulfurizing step.
The present invention is based upon the use of rare earth metals and rare earth metal alloys as desulfurizers. Rare earth metal sulfides oxides and oxysulfides are formed in the melt and float up to the slag. The resulting rare earth metal compounds are insoluble and do not volatilize and form smoke.
Heretofore, rare earth metals have been used in high-strength, low alloy steels to control sulfide morphology in the solid state. The rare earth metals form small sulfide inclusions rather than sulfide stringers in the high-strength, low allow steels. Such small inclusions can be detrimental, however, in electrical steels wherein they interfere with the magnetic properties of such steels. It has been found that if the rare earth metal content (based upon the cerium content) in the melt, is maintained at a level of up to about 400 ppm., and preferably from about 75 to about 250 ppm., the final product will demonstrate both mechanical and magnetic characteristics at least equivalent to those of the typical cold rolled, non-oriented silicon steel desufurized with calcium silicon.